Boiler displays E04 and shuts down; may occur after overheating or poor circulation.

Safety: Allow the boiler to cool completely before touching any internal components. Turn off power at the isolator.

Steps:

• Check system pressure: On the Attack control panel, verify that system pressure is within the recommended range (typically 1.0–1.5 bar when cold). If low, top up via the filling loop following the manual.
• Bleed radiators: Air in the system can cause poor circulation and overheating. Use a radiator key to bleed air from the highest radiators until water flows steadily.
• Verify all valves are open: Ensure isolation valves under the boiler and any zone valves are open and not stuck closed.
• Check pump operation: With power restored, call for heat and listen/feel for the Attack circulation pump running. If it is silent or extremely hot to the touch, it may be seized.
• Reset overheat thermostat: Some Attack models have a manual reset overheat thermostat on the heat exchanger. Press the reset button if accessible (refer to manual). If it trips again, do not keep resetting—find and fix the cause.

Technician required: Persistent E04 indicates a circulation or heat exchanger problem that should be checked by a professional.